FIG. 1 shows an environment with two connected High Voltage Direct Current (HVDC) point to point systems. A first HVDC point to point system comprises a first converter station 1a and a second converter station 1b, with a first HVDC link 5a between the first converter station 1a and the second converter station 1b. Similarly, a second HVDC point to point system comprises a third converter station 1c and a fourth converter station 1d, with a second HVDC link 5b between the third converter station 1c and the fourth converter station 1d. 
The nominal voltage on the first HVDC link 5a is V1 and the nominal voltage on the second HVDC link 5b is V2. If V1 is equal to V2, or almost equal, the two HVDC links 5a-b can be interconnected with a crosslink 2. In this way, power can flow in either direction between the two HVDC point to point systems.
However, when there is a significant difference between nominal voltage ratings of the two HVDC links, there is a need to convert DC power between the two systems.
It would be beneficial if such conversion could occur with low cost and low complexity.